The building blocks of life on Earth may have come from giant clouds of icy dust deep in outer space, astronomers have told an international astrobiology conference in Australia.

For more than a century, scientists have wondered why the 20 amino acids used by all living things are exclusively left-handed. Amino acids are the carbon-based molecules that make up proteins and enzymes, essential for the survival and reproduction of all life.

But if life arose from a random soup of naturally occurring amino acids on Earth billions of years ago, as scientists believe, then there's no reason why they should have same-handedness, or chirality.

"It's been a mystery to science since Louis Pasteur first noticed it in 1848," astronomer Dr Jeremy Bailey of the Anglo-Australian Observatory told the Bioastronomy 2002 conference in Hamilton Island.

"We know there's a large amount of organic material that forms in molecular clouds in space. And we know a lot of this material was present in the early formation of our solar system. But most of it would have been destroyed in the processes that created the planets," he said.

Some scientists suggested the building blocks of life might have hitched a ride on the comets and asteroids that would have routinely bombarded the Earth during its formation 4.6 billion years ago, when our planet was a lifeless and inhospitable place.

Most, however, had considered this unlikely.

One way of creating left-handed amino acids is to blast them with circularly polarised light, a destructive form of ultraviolet light that more easily breaks down right-handed amino acids. This process eventually creates an excess of left-handed molecules.

But there was no known source of circularly polarised light on the early Earth, which had blown this idea out of the water.

That was until a chance discovery by Dr Bailey and his team, who detected circularly polarized light in space: in fact, in interstellar clouds known to be rich in organic molecules.

Comets originating in clouds like these  with more left-handed molecules than right-handed ones  could have crashed onto the Earth. Because they carried an abundance of left-handed molecules, this might explain why life developed a left-handed bias, he said.

"The early Earth would have been bombarded by comets and asteroids, and you could have had a significant amount of this organic material with an excess of left-handed molecules," said Bailey.

Evidence from ancient meteorites appears to bear this out; although rare, those with a high proportion of organic material show a distinct left-handed bias in the amino acids they contain.

In addition, recent laboratory experiments presented at the conference, in which amino acids were blasted with circularly polarised light, also produced an excess of left-handed molecules.

While Bailey said that while this was not definitive proof that the building blocks of life on Earth came from space, he said it's the strongest explanation so far for this strange left-handed bias in living things.